Offshore processing of natural gas or oil associated gas is becoming a more and more attractive alternative to conventional land-based facilities. Marginal gas fields can be economically developed by means of the FPSO concept. Offshore processing plants on floating structures can be production units for liquefied natural gas (LNG), liquefied petroleum gas (LPG), methanol, gas to liquid (GTL), or ammonia. The ability to station the FPSO vessel directly over fields avoids expensive offshore pipelines. In addition, once the existing field is depleted, the production facility can be moved to a new location to continue production.
One of the critical technical challenges for the FPSO concept is the influence of movement, acceleration and static tilt on the performance of distillation and absorption columns, such as demethanizer, deethanizer, depropanizer, glycol-based dehydration system, or acid gas removal absorber columns. For these units, the uniform countercurrent gas and liquid flow is disturbed by motion leading to reduced heat and mass transfer. Their efficiency is thus, often influenced by the deviations from the vertical position.
Though amine-based acid gas removal is a well proven technology for onshore applications, offshore conditions may result in not contacting some of the untreated or insufficiently treated feed gas due to movement or tilt of the absorber. The problem is especially severe for floating LNG FPSO, since the liquefaction process requires CO2 to be reduced to less than 50 ppm to avoid risk of the freeze out and blockage of heat exchangers at cryogenic temperatures. Even a small bypass may have an adverse impact on liquefaction system operation with off-specification (off-spec.) treated gas having an excess of CO2 exceeding about 50 ppm, and may require the process to shut down resulting in production losses.
Moreover, various studies have established that distillation or absorption column movement due to sea conditions can negatively affect separation efficiency in the column. In a 50 mm diameter distillation column, for example, a 50% reduction in efficiency at an inclination of 2.5 degrees was reported. In addition, a 16% reduction in efficiency at an inclination of 1.2 degrees for a 2 meter diameter one or two-pass tray column, and a 31% reduction in efficiency at an inclination of 1 degree for a 330 mm diameter one-path tray column were reported.
These reported reductions in separation efficiency will have a significant impact on the equipment performance, and eventually the reliability of the operation. Availability of production will suffer due to process shut down triggered by the off-spec treated gas from the acid gas removal unit. Equipment may need to be over-sized to offset the reduction in separation efficiency at higher sea states. Alternatively, proprietary equipment may need to be used to alleviate the uneven distribution problem.